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Technical Articles

Database

Troubleshooting MySQL Server Startup Failures

If the MySQL service fails to start, you need to check the cause of the error log location before processing it in a targeted manner. 1. Check the error log path such as /var/log/mysql/error.log, and pay attention to keywords such as "Unabletolock" and "Addressinuse"; 2. Check the occupancy of port 3306, and adjust the memory parameters such as innodb_buffer_pool_size; 3. Confirm that the data directory permissions are mysql users, check for the .pid file residue or InnoDB corruption, and enable innodb_force_recovery if necessary; 4. Ensure the initialization operation is completed and use mysqld--validate-con

Aug 04, 2025 pm 01:27 PM

How to Optimize MySQL Server Settings for High Traffic Websites?

TunekeyMySQLconfigurationvariablesbyadjustinginnodb_buffer_pool_sizeto70%ofRAMandsettinginnodb_buffer_pool_instancesto8forconcurrency,2.optimizeschemaandindexesusingproperdatatypes,strategicindexing,andenablinginnodb_file_per_table,3.enableslowqueryl

Aug 04, 2025 pm 01:25 PM

SQL Time Series Data Handling

Key techniques for processing time series data include grouping statistics by time, aligning fill missing values, difference and growth calculations, and simple predictions. 1. Group statistics by time can be achieved by using functions such as DATE_TRUNC, DATE_FORMAT, etc. in combination with GROUPBY; 2. Aligning and filling missing values can be used to generate a complete time range and connect left to the original data, and fill the empty values with COALESCE; 3. Difference calculation uses LAG() or LEAD() window function, and combines COALESCE to process the initial row; 4. Simple prediction can be used to move average or linear interpolation, and complex models are recommended to combine Python or R.

Aug 04, 2025 pm 01:16 PM

How does the COALESCE function work in SQL?

COALESCEreturnsthefirstnon-NULLvaluefromalistofexpressions,enablingcleanerhandlingofmissingdata;1.Itevaluatesargumentslefttorightandreturnsthefirstnon-NULLvalue;2.Itiscommonlyusedtosubstitutedefaults,suchasreplacingNULLphonenumberswith'Nophone'or0inr

Aug 04, 2025 pm 01:15 PM

Always Encrypted in SQL Server: Protecting Sensitive Data

AlwaysEncryptedinSQLServerenhancesdatasecuritybyencryptingsensitivedataattheclientside.1)Setuprequiressupportedclients,CMK,CEK,andtoolslikeSSMS.2)Usedeterministicencryptionforsearchablecolumnsandrandomizedforhighersecurity.3)Securelymanageandrotateke

Aug 04, 2025 pm 01:01 PM

Can you explain the different types of constraints in SQL, such as NOT NULL, UNIQUE, and CHECK?

SQL constraints are used to ensure the accuracy and integrity of data, mainly including: 1. The NOTNULL constraint requires that the field must have a value to prevent null values; 2. The UNIQUE constraint ensures that the field value is unique, allowing a NULL but prohibiting duplication of non-null values; 3. The CHECK constraint forces the field value to meet the specified conditions, such as age ≥18; 4. The PRIMARYKEY constraint is a unique identifier, and has both NOTNULL and UNIQUE characteristics; 5. The FOREIGNKEY constraint maintains the integrity of references between tables to prevent invalid foreign key references; 6. The DEFAULT constraint provides a default value for the field, and is automatically filled if not specified during insertion. These constraints jointly guarantee data reliability and avoid duplicate, invalid or orphaned records. They are database

Aug 04, 2025 pm 12:56 PM

database SQL約束

What is the difference between a temporary table and a table variable in SQL?

Tablevariableshavetighterscopeandshorterlifetime,limitedtothecurrentbatchorprocedure,whiletemporarytablescanpersistacrossbatchesandsessions.2.Temporarytablesarestoredintempdbwithfullindexandstatisticssupport,enablingbetterqueryoptimizationforlargedat

Aug 04, 2025 pm 12:34 PM

How can you select unique values from a column in SQL?

ToselectuniquevaluesfromacolumninSQL,usetheDISTINCTkeyword;1.PlaceDISTINCTafterSELECTandbeforethecolumnnametoremoveduplicates;2.Formultiplecolumns,listthemafterDISTINCTtogetuniquecombinations;3.RememberDISTINCTtreatsNULLsasequalandappliestotheentirer

Aug 04, 2025 pm 12:33 PM

Understanding MySQL Explain Plan Output for Query Analysis

To analyze the performance of MySQL query, you can determine whether the query is efficient by using the execution plan output by the EXPLAIN command. 1. Focus on the type field to avoid ALL type full table scanning, ideally ref, eq_ref or range; 2. Check the key field to confirm whether the appropriate index is used. If it is not used, it is necessary to optimize the SQL or index design; 3. The smaller the value of the rows field, the better, and the estimated number of scanned rows can improve performance; 4. When Usingfilesort or Usingtemporary appears in the Extra field, it should be optimized, such as adding indexes or adjusting queries. Mastering these core fields and combining them with actual scenario analysis can effectively discover and solve query performance bottlenecks.

Aug 04, 2025 pm 12:22 PM

Automating Database Tasks with SQL Agent Jobs

SQLServerAgent can be used to automate database tasks to improve management efficiency. 1. You can set up a timed backup database. It is recommended to perform a full backup at 2 a.m. every day and store the backup files in a separate partition. 2. Old data can be cleaned regularly, such as deleting or archived data from 6 months ago every month, and it should be done in batches when deleting it. 3. Mail notifications can be configured to send execution results to the administrator when the task is completed or failed. 4. Statistical information and index reconstruction can be automatically updated. It is recommended to rebuild high-fragment indexes every week and update the statistical information of frequent change tables every day.

Aug 04, 2025 pm 12:17 PM

How do you use the HAVING clause to filter groups in SQL?

Use the HAVING clause to filter grouped data after GROUPBY, especially when the conditions involve aggregation functions such as COUNT(), SUM(), AVG(); 2. Unlike the WHERE clause, WHERE is used to filter a single row before grouping, while HAVING is used to filter groups based on the aggregation result after grouping; 3. HAVING must be placed after GROUPBY, and column alias in SELECT cannot be used, and the aggregate expression must be repeated; 4. WHERE and HAVING can be used at the same time, the former filter the original row and the latter filter the grouping results; 5. Common application scenarios include finding customers whose order number exceeds the specified value, departments whose average salary is higher than a certain value, or excluding groups with NULL values; 6. Total

Aug 04, 2025 pm 12:12 PM

sql HAVING子句

What is the difference between LEFT JOIN and INNER JOIN in MySQL?

INNERJOINreturnsonlyrowswithmatchesinbothtables,excludingunmatchedrows;2.LEFTJOINreturnsallrowsfromthelefttableandmatchingrowsfromtherighttable,withNULLsforunmatchedrighttablecolumns;useINNERJOINtogetonlyrelateddataandLEFTJOINtoincludealllefttablerow

Aug 04, 2025 am 11:42 AM

mysql join

how to use window functions in mysql

MySQL supports window functions since version 8.0, which greatly improves the processing efficiency of complex queries. 1. Window functions can calculate a set of rows without merging and paralleling, such as ranking, cumulative statistics, etc.; 2. Common functions include ROW_NUMBER(), RANK(), DENSE_RANK(), SUM()OVER(), AVG()OVER(), LAG(), LEAD(); 3. When using it, you must match the OVER() clause, and PARTITIONBY and ORDERBY can be specified; 4. Typical application scenarios include ranking query (such as TopNperGroup), cumulative statistics (such as RunningTotal), and context comparison (such as month-on-month analysis

Aug 04, 2025 am 11:32 AM

What is an execution plan in Oracle and how to read it?

AnexecutionplaninOracleisaroadmapshowinghowthedatabaseexecutesaSQLquery,revealingoperationsliketablescans,indexusage,joins,andsorts;itmattersbecauseithelpsidentifyinefficientindexuse,unnecessaryfulltablescans,andcostlyoperations,enablingSQLperformanc

Aug 04, 2025 am 11:26 AM